In this study sensory function and temperament were compared in infants varying in gestational and medical status at birth. The infants who were studied included low medical risk preterms at their expected date of delivery, healthy full-term neonates, and post-term neonates. Sensory function was evaluated by measures of response to novelty, level of responsiveness, and response decrement collected when the infants were responding to tactile, auditory, and visual stimulation. Temperament was assessed on the basis of the spontaneous behaviors of the babies, their responsiveness to soothing when they cried, and their responsiveness during a neurological examination. Temperament was described in terms of state organization, irritability, soothability, and activity level. Preterms were different from full-terms on auditory, visual and temperamental measures. Post-term infants differed from full-terms on some temperamental measures.